To make sorrel pesto:
Combine garlic, nuts and cheese in food processor and pulse until coarse. Add sorrel leaves and lemon juice and pulse until combined. While processor is running, stream in olive oil until desired consistency is reached. Season with salt and pepper and set aside.
To make pizza:
Preheat oven to 450°. Place a jellyroll pan (approximately 10x15") inside to heat.
Heat 1 tablespoon of oil in medium sauté pan over medium heat. Add onion and sauté, stirring occasionally, until onion starts to brown. Remove from heat and cool slightly. Season with pepper. Meanwhile, place potato slices in a bowl. Coat with 1 tablespoon of oil and sprinkle with salt. Set aside.
Remove preheated pan from oven and drizzle with olive oil over entire surface, making sure to coat well. Turn out pizza dough onto pan and stretch it out so it so it comes to the rim on all sides. Scatter grated cheese evenly over dough, followed by potato slices, onions and goat cheese. Bake until potatoes are tender, cheese is melted and underside of crust is slightly browned and crispy, about 15 to 20 minutes.
Remove from the oven and drizzle with sorrel pesto. Cut into pieces and serve immediately.